1. Field of the Invention
The present invention generally relates to an electronic technique for identifying a fluid and determining its level in a container, and more particularly, to the application of impedance spectroscopy to fluid handling probes such as those employed by automated chemical and immunoassay analyzers.
2. Background
Current fluid level detection technologies typically employ electronic techniques to measure changes in conductance or capacitance as an analyzer's fluid transfer pipette contacts a fluid surface. Each method effectively identifies fluid surfaces, but each also bears significant inherent disadvantages within the context of automated chemical and immunoassay analyzers.
Conductance techniques require two conducting points, both of which are typically inserted into the fluid of interest. These are typically incorporated into an assembly comprised of a conductive pipette probe positioned in close proximity to a wire-like conductor. Thus, a circuit is completed when the tip of the assembly is inserted into a fluid. Unfortunately, such configurations are mechanically delicate, difficult to clean and are prone to contaminate fluids into which they are inserted.
Capacitance techniques require two conducting surfaces. In contrast to the conductance technique, capacitance techniques require the insertion of only one conductor into the fluid of interest. This is typically the pipette probe itself, so the resulting physical configuration is often more robust than those employed for conductance measurements. However, as the second conductor, capacitance measurements require placement of an effective ground plane in contact with the fluid container in order to achieve sufficient sensitivity. As such, they work poorly, if at all, with fluid containers positioned at any distance from a ground plane. Unfortunately, this problematic situation frequently arises when automated analyzers need to handle small volume containers known as tube top cups. Further, capacitance-based methodologies are vulnerable to changes in ambient conditions including atmospheric variations and the proximity of nearby objects such as laboratory personnel.
In addition to these drawbacks, both capacitance and conductance techniques lack the ability to identify fluids, and are unable to distinguish fluid surfaces from air bubbles situated atop them. The inadequacy renders analyzers vulnerable to fluid sampling failure if the pipette aspirates air bubbles instead of fluid.
U.S. Pat. No. 5,275,951, Liquid level sensing Method and Device, describes an immunochemistry analyzer that employs a bi-static RF transmitter/receiver pair for determination of liquid levels in conjunction with electronic circuitry to process received signals.
U.S. Pat. No. 4,977,786, Capacitive Liquid Level Sensor, describes a commonly used level sensor. In this patent, probe and fluid are part of a circuit that generates a phase difference depending on the capacitance value of the probe and the fluid it contacts. The patent contemplates using an analog electronic means, which generates high frequency oscillations, for comparison purposes.
U.S. Pat. No. 5,049,826, Liquid Level Sensing Apparatus for use in Automatic Chemical Analysis, describes a level sensor that uses a balanced bridge with a probe as part of the bridge. When the probe touches the sample, the bridge is no longer balanced and a difference in the bridge is measured. The system described in the patent is run in the kHz region, and uses an analog electronic means for both comparison and generation of high frequency oscillations.
U.S. Pat. No. 5,083,470, Capacitive Liquid Level Sensor, describes a level sensor that is similar to that described in U.S. Pat. No. 4,977,786. In U.S. Pat. No. 5,083,470, an inductive coil is placed so that the probe protrudes through the coil so as to reduce the chance of false alarms.
U.S. Pat. No. 5,627,522, Automated Liquid Level Sensing System, describes a level sensor which is similar to that described in U.S. Pat. No. 5,275,951. U.S. Pat. No. 5,627,522 describes a bi-static system and not a reflection measurement.